As you know, Bob, I do stuff and then I write about it. Here are some of my thoughts about clay.
Brother-the-younger built a kiln. It makes pottery into bisque. It *may* make glazes + bisque into watertight pottery, but that remains to be seen. Brother-the-younger read up on clay and dug some out of a creek bank in the valley. He picked the wrong creek but asked me for input following his less-than-optimal experiences and I directed us to the right creek. Go me!
The both of us wrote to Aunt Charlotte, who has a fine arts degree wherein she apparently learned rather a lot about pottery. Aunt Charlotte wrote back with sort of a meta-dump of her pottery experience, being cut from the same 'more information is better than less' cloth as the rest of us. It is preserved here for the edification of us all:
I have lots of suggestions here. Pots breaking in the kiln result from a variety of errors, and sometimes, they just happen.
When you mine your clay, dry it out completely in small fistfuls. A sunny spot in a driveway is good, but be mindful of rainy weather. This will take a couple of days. Pulverize it and sieve it to remove stones. Stones cause breaks in the firing process. Now you have a fine, powdered clay. Save some of the powder; you'll need it for your glaze. Pour some powder into a bucket of water and allow it to set for another day. Pour off the excess water and set the clay out on the driveway again to begin drying. This may take a day, but you need to check on it. When the clay is firm enough to hold its shape, begin kneading it like making bread, until it's thoroughly mixed and plastic. You can do this on a counter top, or better yet, on a thick slab of dried plaster. (Do not let any plastic mix with the clay - it will cause the clay to break too.) Wrap your clay in heavy plastic trash bags and be sure it is 100% air tight. It will keep for a month or more. You can also put it in a plastic paint container for extra insurance, but it's not necessary. If the clay molds, it's not a problem. Mold does not survive firing. You can try adding finely ground sand and vermiculite into the clay when you're kneading (wedging) it to make your clay a bit stronger. Vermiculite can be found in hardware stores sold separately in bags, and it is added to potting soil for aeration. Look in the houseplant section. Vermiculite is silica popcorn - all natural.
Make your vessels no thicker than 1/2" anywhere. Myth has it that air pockets cause pots to break in the kiln. That's true enough, but the greater problem is clay walls that are too thick. Heated, excited water in the clay body cannot escape the clay's interior and causes breakage.
When you make your pots, dry them completely. To test for "doneness", close your eyes, pick up your pot, and feel for a sensation of coolness on the bottom. If the pot is cool, there's still water in the clay. The clay must be "bone dry" to fire successfully. Usually, in an average room at home, this takes 10-12 days. Don't try to force the drying process with an oven or hair dryer - you'll have uneven drying. You can turn the vessel over from time to time to let the bottom dry too. Bone dry clay is ultra fragile. Be careful!
I have learned the hard way that clay shatters in kilns when flames touch the pots. Your pots have to be heated indirectly by hot air. I don't know what you're setting your pots on, but you might try using an old grill or oven shelf set up on bricks inside your kiln. Using that, the heat will surround the pots on all sides and will fire more evenly. Now, you need to surround your pots with something that prevents flames from getting to them. Old sheet metal, like cookie sheets or license plates will do. Form a teepee around the pots, but leave small spaces for air to get through. You can set the metal directly against the pots. I would shop for metal cookie sheets or old cook pots at the local Salvation Army store. You can also buy and use earthenware, unglazed plant pots, the red kind, from your local hardware store. Set each pot inside the larger, upside down red pot. Stack your fuel around the pots and begin firing. You should fire slowly, building up to the hottest temperature after about six hours. You will need to continue to add fuel as you go. You want a red-orange glow sustained for about an hour. Cool down overnight.
You have two choices with surface treatment. You can burnish your pots with the back of a spoon when the pots are leather hard, that is, when they are somewhat dry and inflexible, but still cool to the touch. Leather hard clay looks like the original clay mixture; it hasn't lightened in color yet. Leather hard clay is just past the point where it's flexible. It's stiff. You can use a rag with a bit of cooking oil on it to wipe down the pot as you burnish. Burnishing takes awhile. You want to really compress the clay by going over and over the surface. Clay molecules are like shingles - they overlap. The more you burnish, the tighter the surface. Burnished pots do not hold water though. Early cultures applied beeswax inside and out their pots to make them waterproof. But every culture in the world eventually discovered ash glazes.
Ash glazes have to be applied after the pot has been fired once already, and they make the vessel waterproof and food safe. Apply the ash glaze, dry overnight. Fire again. Don't allow the pots to touch or they will melt together. Glaze the inside of the vessel, but not the bottom, or else it will melt to anything it sets on.
To make an ash glaze, collect your old wood ashes from your fireplace or your last kiln firing. Sift the ash to remove bits of charcoal. Pour the ash gingerly into a bucket of water - you don't want to breath this dust - highly alkaline. Add some powdered, sifted clay that you made earlier. The ratio should be 1 part ash to 3 parts clay powder by weight. Stir, pour off the excess water and allow the glaze to thicken to the consistency of buttermilk. Brush it on your pot and allow to dry overnight before firing again, the same way as before. Different kinds of wood make different colored glazes, but mostly, they are clear to gray.
You can go to YouTube or Google and finds loads of information on pit firing using wood and making and applying ash glazes or burnishing. Do not believe potters who pile the wood directly on top of their pots. They are using clay bodies especially formulated for temperature shocks. Using local clay, you have to heat your pots indirectly.
It sounds to me like you are using a Raku method - removing the pot at maximum temperature and sinking it into flammable material such as leaves or sawdust. This works best with glazed wares. A metal trashcan with a lid works well. Put the organic debris in the trashcan, bury the hot pots in there, and slam on the lid. The lack of oxygen makes for beautiful effects. This might be fun with the ash glaze. There are recipes for Raku glazes online. You'll need to order supplies from a ceramic supply house. The Raku process also requires a special clay body to withstand the temperature shock. I used to buy the clay already made to make sure I had something to show for all my effort. It may not work with your local clay, and the temperature shock may be at the root of your problem.
You can get some blackening of the surfaces by filling and surrounding your pots with sawdust, or wrapping them with several layers of newspaper. You get interesting smokey effects on the surface this way.
You are right about the perils of adding salt to firing pots. We used to do this at SIU. Len and I lived in a little house along the railroad tracks. The salt kiln was about 1/4 mile from our house and along the same tracks. When they salt fired at night, we would about choke to death from the gases. Finally, the city closed down the operation and SIU had to build a salt kiln out in the country away from town.
When Aaron was in 5th grade, he had to make a Colonial American handmade product for a history grade. We made clay pots and fired them in our back yard. We fired them at a very low temperature in our barbeque grill using wood. It took us an afternoon to fire. The clay did not vitrify, that is to say, to mature to a true ceramic product, but his teacher didn't know the difference. He got an A and got into the newspaper with his little pots. I know for a fact that I had more fun than he did.
Apparently graduate school in fine arts includes rather a lot about pottery. So, armed with that infodump (and a sorta-sciency book about ash glazes that BtY bought), we set forth.
I started with a shovel and a creek bed. I dug up the clay. I dried out the clay and pulverized it and ran it through an old window screen to remove teeny thready tree roots and itty-bitty rocks, of which there were not many. (We are nothing if not high tech around here, I must say.) And then I took my clay dust into my house and mixed it up with water (a portion of it) to reconstitute it into clay, in a mixing bowl. I used the one I also used for food but it's stainless steel and will not suffer any damage from dirt. I messed with my clay on the kitchen counter, which was not nearly as messy as you are probably thinking. (Again, just dirt. Washes off. Does not have that many germs.)
I have made pots. They totally suck, being little, lopsided, unbalanced, heavy-looking pots made by coiling little snakes of clay around and around, the same way that eight year olds do. My pots look pretty much like they were made by an eight year old except that I am forty-three and can understand how fully my pots suck. Potter satisfaction = low. (At least stuff I make on the lathe is fucking round.) I am a beginner and I understand that a lot of my sucking is the fact that I haven't had any occasion to practice my pot-making technique since I was Play-Doh age. The rest of my suckage is comprised of my complete lack of artistic talent. (I can kind of manage craft, but I don't have any chops at all for "art". One's delusions on this front get quashed pretty early when one's family includes fine-arts-degreed persons.)
As might be expected, I learned some things en route to "shitty pots that look like they were made by an eight year old". Here is what I learned.
1. When you go to make stuff with your clay, there is a degree-of-wet that is proper and a degree-of-wet that is WAY TOO MUCH. The wetness needed is less than you probably think. Your instincts are going to be wrong. But, it's OK. You mix the clay and then you take it out and mash it around with some of the leftover powdered clay and it will get to be the right amount of wet that way. You want it to be only a little wet and not sticking to everything in huge gobs of sticky icky.
2. If you heat with a woodstove, the air in your house is going to resemble that of the Sonoran desert. It is helpful to put a damp paper towel or two over your big lump o' clay to keep it from getting too dry while you're screwing around with your teensy snakes of clay.
3. A drywall knife is a very helpful tool for moving around clay, cutting off lumps of clay, etc. A 6" knife is probably fine. (We do not have pottery stores or "art" stores around here, but we do have a halfway decent hardware store.)
4. Keep a smallish amount of liquidy water+clay handy to smear things together. Use a finger or two, it doesn't take a whole lot.
5. Set aside a five gallon bucket half full of warm water for clean-up so that you don't run clay down the drain. Septic systems do not like clay. You can throw the clay-y water outside in the yard when you're done.
6. If you stick your pot-in-progress to the countertop, it will not come off the countertop very well. Use parchment paper or a ziplock baggie to set it on -- they are relatively nonporous and pretty flexible so that you can peel 'em off when the pot sets up a little.
If the little misshapen clay pots get dry enough over the holiday weekend, we're going to fire 'em in the kiln with Adult Beverages to hand. (Also while kiln-ing, we have planned the following experiment: Can we melt (empty) glass beer bottles into interesting shapes and/or a more-liquid nature? We really know how to have a good time at chez moi.)